Cardiovascular disease is a major contributor to patient illness and mortality. It also is a primary driver of health care expenditure, costing billions of dollars each year in the United States. Heart failure is the final common expression of a variety of cardiovascular disorders, including ischemic heart disease. It is characterized by an inability of the heart to pump enough blood to meet the body's needs and results in fatigue, reduced exercise capacity and poor survival. Heart failure results in the activation of a number of body systems to compensate for the heart's inability to pump sufficient blood. Many of these responses are mediated by an increase in the level of activation of the sympathetic nervous system, as well as by activation of multiple other neurohormonal responses. Generally speaking, this sympathetic nervous system activation signals the heart to increase heart rate and force of contraction to increase the cardiac output; it signals the kidneys to expand the blood volume by retaining sodium and water; and it signals the arterioles to constrict to elevate the blood pressure. The cardiac, renal and vascular responses increase the workload of the heart, further accelerating myocardial damage and exacerbating the heart failure state. Accordingly, it is desirable to reduce the level of sympathetic nervous system activation in order to stop or at least minimize this vicious cycle and thereby treat or manage the heart failure.
Hypertension, or high blood pressure, is a major cardiovascular disorder that is estimated to affect 65 million people in the United States alone. Hypertension occurs when the body's smaller blood vessels (arterioles) constrict, causing an increase in blood pressure. Because the blood vessels constrict, the heart must work harder to maintain blood flow at the higher pressures. Although the body may tolerate short periods of increased blood pressure, sustained hypertension may eventually result in damage to multiple body organs, including the kidneys, brain, eyes and other tissues, causing a variety of maladies associated therewith. The elevated blood pressure may also damage the lining of the blood vessels, accelerating the process of atherosclerosis and increasing the likelihood that a blood clot may develop. This could lead to a heart attack and/or stroke. Sustained high blood pressure may eventually result in an enlarged and damaged heart (hypertrophy), which may lead to heart failure.
Hypertension is a leading cause of heart failure and stroke, is the primary cause of death for tens of thousands of patients per year, and is listed as a primary or contributing cause of death for hundreds of thousands of patients per year in the U.S. Accordingly, hypertension is a serious health problem demanding significant research and development for the treatment thereof. Hypertension remains a significant risk for patients and challenge for health care providers around the world despite improvements in awareness, prevention, treatment and control over the last 30 years. Patients with hypertension are encouraged to implement lifestyle modifications including weight reduction, adopting the DASH eating plan, reducing dietary sodium, increasing physical activity, and limiting alcohol consumption and smoking. A large number of pharmacologic treatments are also currently available to treat hypertension.
An improved approach for treating hypertension, heart failure and/or other cardiovascular disorders has been developed. Baroreflex Activation Therapy (“BAT”) utilizes electrical, mechanical, chemical, and/or other means of stimulation to activate one or more components of a patient's baroreflex system, such as baroreceptors. Baroreceptors are sensory nerve ends that are profusely distributed within the arterial walls of the major arteries, as well in the heart, aortic arch, carotid sinus or arteries, and in the low-pressure side of the vasculature such as the pulmonary artery and vena cava. Baroreceptor signals are used to activate a number of body systems which collectively may be referred to as the baroreflex system. Baroreceptors are connected to the brain via the nervous system, allowing the brain to detect changes in blood pressure, which is indicative of cardiac output. If cardiac output is insufficient to meet demand (i.e., the heart is unable to pump sufficient blood), the baroreflex system activates a number of body systems, including the heart, kidneys, vessels, and other organs/tissues. Such natural activation of the baroreflex system generally corresponds to an increase in neurohormonal activity. Specifically, the baroreflex system initiates a neurohormonal sequence that signals the heart to increase heart rate and increase contraction force in order to increase cardiac output, signals the kidneys to increase blood volume by retaining sodium and water, and signals the vessels to constrict to elevate blood pressure. The cardiac, renal and vascular responses increase blood pressure and cardiac output, and thus increase the workload of the heart. In a patient suffering from heart failure, this further accelerates myocardial damage and exacerbates the heart failure state.
One of the first descriptions of treating hypertension through baroreceptor stimulation appears in U.S. Pat. No. 6,522,926 to Kieval et al., which discloses devices and methods for stimulating or activating baroreceptors or the baroreflex system to regulate blood pressure and/or treat other cardiovascular disorders. Generally speaking, a baroreceptor activation device may be activated, deactivated or otherwise modulated to activate one or more baroreceptors and induce a baroreceptor signal or a change in the baroreceptor signal to thereby affect a change in the baroreflex system. The baroreceptor activation device may be activated, deactivated, or otherwise modulated continuously, periodically, or episodically. The baroreceptor activation device may utilize electrical as well as mechanical, thermal, chemical, biological, or a combination thereof to activate the baroreceptor. The baroreceptor may be activated directly, or activated indirectly via the adjacent vascular tissue. Activation of this reflex increases afferent electrical signals through the carotid sinus nerve (Hering's nerve, a branch of the glossopharyngeal nerve, cranial nerve IX) to the medullary brain centers that regulate autonomic tone. Increased afferent signals to these medullary centers cause a reduction in sympathetic tone and an increase in parasympathetic tone. This results in lower heart rate, reduced sodium and water reabsorption by the kidney resulting in a diuresis, relaxation of the smooth muscle in the blood vessels which results in vasodilatation and a reduction in blood pressure. Thus, peripheral activation of the baroreflex results in a physiologic response whereby blood pressure is controlled by mechanisms determined by the integrative action of the central nervous system action on all peripheral organs and blood vessels.
The process of implanting a baroreflex activation device, such as an electrode assembly, for delivering baroreflex therapy is known as mapping—positioning the assembly such that the electrodes are properly situated against the wall of a vessel containing baroreceptors, and securing the electrode assembly to the artery so that the positioning is maintained.
Mapping adds to the overall procedure time due to adjusting and re-adjusting the position of the electrode assembly during implantation. Present-day procedures involve positioning and holding the electrode assembly in place with forceps, hemostat or similar tool while applying the stimulus and observing the response in the patient. Movement by as little as 1 mm can make a medically relevant difference in the effectiveness of the baroreceptor activation.
The positioning is a critical step, as the electrodes must direct as much energy as possible toward the baroreceptors for maximum effectiveness and efficiency. The energy source for the implanted baroreflex stimulation device is typically an on-board battery with finite capacity, and it is desirable to provide a lower energy source to ensure patient safety. A high-efficiency implantation will provide a longer battery life and correspondingly longer effective service life between surgeries because less energy will be required to achieve the needed degree of therapy. As such, during implantation of the electrode assembly, the position of the assembly is typically adjusted several times during the implantation procedure in order to optimize the baroreflex response. One example of mapping methods and techniques for implanting electrodes is disclosed in U.S. Pat. No. 6,850,801 to Kieval et al.
Current generation implantable baroreflex therapy systems, such as described in U.S. Pat. No. 8,437,867 to Murney et al., include an implantable pulse generator and associated circuitry contained within a hermetically sealed housing, an elongate flexible electrical lead connectable to the housing, and a monopolar electrode structure coupled with the electrical lead. As used herein, the words “housing,” “enclosure,” “case” and “can” are synonymous when used to refer to the housing of the implantable pulse generator. At least a portion of the housing is conductive for use as an electrode in conjunction with the monoploar electrode structure on the lead. Such a housing may be referred to as an active can.
The mapping procedure may be performed with some or all components of the implantable baroreflex therapy system. While it would be possible to conduct the mapping procedure utilizing specialized equipment retained by the hospital or clinic, such as one or more temporary and/or reuseable leads which are connectable to an external pulse generator, this equipment increases the overall cost, complexity and time of the mapping procedure. Thus, it is desirable to utilize as many of the implantable system components as possible for the mapping procedure.
Further, it is desirable to perform the mapping procedure prior to fully implanting the therapy system. Creating a pocket in the chest of the patient placement of the implantable pulse generator may require use of anesthetics which can impact the mapping procedure by altering the patient's response, and leaving such a pocket open during the mapping procedure increases the risk of infection. Thus it is desirable to refrain from creating a pocket in the chest of the patient for placement of the implantable pulse generator, and/or refrain from tunneling a path for the lead from the pocket to the electrode implant site, until it has been confirmed through the mapping procedure that a suitable patient response has been obtained. In the event a suitable response from the patient cannot be obtained during the mapping procedure, the implant procedure may be postponed or terminated.
However, because current generation implantable baroreflex therapy systems utilize a monopolar electrode structure in combination with an active can (the housing of the implantable pulse generator is electrically conductive to create a return path from the electrode structure), a problem exists for performing the mapping procedure without creating a pocket in the chest of the patient for the implantable pulse generator.
A need therefore exists for more cost- and time-effective devices and methods for performing a mapping procedure as part of implanting a baroreflex therapy system.